Method of heating and forming powdered metals



y 1957 L. J. BONIS ETAL 3, 3

METHOD OF HEATING AND FORMING POWDERED METALS Filed Sept. 29, 1964 INVENTORS LASZLO d. BONIS OLE-ANDREAS SANDVEN BERNARD MANNING BY M ATTORNEYS United States Patent 3,331,686 METHOD OF HEATING AND FORMING POWDERED METALS Laszlo J. Bonis and Ole A. Sandven, Brookline, and Bernard Manning, Boston, Mass., assignors to llikon Corporation, Natick, Mass, a corporation of Delaware Filed Sept. 29, 1964, Ser. No. 400,259 7 Claims. (Cl. 75-226) Our invention concerns an improved apparatus for and a method of the heating and consolidation of metal powders. In particular, our invention relates to a means of hot forming articles from powdered metals containing solid lubricants which may or may not be decomposed at elevated temperatures. More particularly, our invention concerns a means of hot forging or coining from metal powders articles with superior mechanical properties and of enhanced density approaching theoretical density. This invention in a continuation-in-part of our patent application Serial No. 311,831, filed Sept. 26, 1963, now Patent No. 3,248,215.

The techniques of powder metallurgy have permitted powdered metals to be cold or hot pressed into a variety of articles'such as gears, brushes, sleeves, valves, bearings, shafts, bushings, etc. Sintering of the metal particles either during or after compaction in a mold is accomplished by heating at elevated temperatures below the fusion point of the powder for suflicient time to promote incipient fusion and the destruction of the grain boundaries or until diffusion occurs between the metal particles, Sintering and elevated temperatures can be accomplished by employing the mass of metal powders in an inductive relationship with a coil connected to a high frequency generator. The currents set up within the mass of metal powder bring about a rapid rise in temperature of the metal mass and the sintering of the mass.

Although the use of a conductive high frequency helix about the electrically conductive mass of powdered metal is satisfactory for some purposes it has disadvantages which inhibit wide commercial acceptance of this method.

In many cases a dry lubricant must be admixed with or impregnated into the powder metal mass to provide a self lubricating article, containing finely divided solid phase particles of dry lubricants uniformly dispersed through the solid sintered matrix. Many of the commercially acceptable dry lubricants which are desirable to blend with the metal powder prior to sintering are chemically degraded at a progressively rapid rate as the temperature of the sintering increases. For example, molybdenum disulfide completely oxidizes above about 750 F., 'while tungsten disulfide is completely oxidized at above about 850 F. in an oxidizing atmosphere. Present high frequency techniques due to the rapid temperature rise and uneven temperatures within the metal mass often tend to degrade and reduce the effectiveness of heat sensitive dry lubricants.

Additionally, long heating time and elevated temperatures obtained with high frequency energy also encourages reactions between the ambient gases such as nitrogen and oxygen with the metal mass. The formation of metal matrix oxides is generally undesirable in powder metallurgy, since they hinder consolidation to high density articles with superior mechanical properties.

The degradation of heat and temperature sensitive lubricants and the reaction of the metal mass with atmospheric gases is further promoted by uneven heat distribution and temperatures within the metal mass especially with larger metal masses. Significant uneveness in heat distribution often occurs and the lack of uniform temperatures through the compacted mass leads to unsatisfactory sintered articles with poor hardness and density characteristics. For example, particularly with larger masses of electrically conductive metal powders heated by the conventional use of continuous radio frequency energy sufficient time must be allowed to permit an even temperature of the mass. During this time lubricants are often degraded and overheating of the external or other portion of the mass often occurs with undesirable oxidation of the metal powder. Even if these difficulties are avoided or are insignificant, the conventional hot sintering and hot pressing techniques conventionally employed provide compacted metal articles which require additional heat treatment and quenching to obtain acceptable hardness and ductility properties. Further, conventional means of hot pressing metal powders permits articles having a density in the range of to percent of the theoretical density. A conventional method of forming metal powders into compacted articles would comprise pretreatment of the metal powder composition such as the mixing and blending of various metal powders and lubricants to a uniform mix and then forming the powder mix into a green compact mass such as by cold pressing, slip casting or otherwise consolidating the powder to the green compact using little, if any, heat. The green compact is then formed into the desired end article by sintering or hot pressing at elevated temperatures, such as by employing a RF coil in an inductive relationship with the green compact and heating and compacting the green compact in a mold.

It is therefore an object of our invention to provide a means of forming powdered metals into articles of superior hardness and high density characteristics.

It is also an object of our invention to provide apparatus for the hot impact consolidation of the metal powders.

A further object of our invention is to provide improved means for forming powdered metal articles of high density containing temperature sensitive dry lubricants.

Other objects and advantages of our invention will be apparent to those skilled in the art from the following detailed description of our invention taken in conjunction with the accompanying drawing wherein:

The figure is a partially schematic and sectional view of an apparatus for heating by a high frequency coil and compacting in a die a green mass of metal powder.

We have found that compacted articles may be prepared from powdered metals, which articles have densities of about percent or more of theoretical density, by employing pulsating high frequency energy to obtain rapid, even-working temperatures and then impact-forming the heated work piece. In our embodiment a green powdered metal compact is prepared in the conventional manner. This green compact is placed in an inductive relationship within a high frequency working coil. The compact is then rapidly heated to a hot working temperature such as above the normal recrystallization temperature of the material by the use of power pulses of sufficiently short duration so that even temperatures are obtained without overheating or causing grain growth or excessive oxidation of the material.

The pulses are continued for a period of time sutficient to bring the green work piece to a forming tem erature, usually within 1 to 10 seconds or more, depending upon the size and mass of the workpiece. The power pulses employed should not be confused with the frequency of the energy source which is normally in the 50 to 500 kilocycle range. The heated workpiece is then impact forged in a die set employing a single stroke operation with sufficient energy to form the desired article. The work piece may be heated separtely and rapidly transferred to a die set orpreferably heated in an impact forging position so that the possibility of oxidation prior to coining or forging is further reduced. One method is to heat the work piece by RF heating in the die set with the green work piece suspended between the die and the working plunger and to forge the hot piece through the RF working coil surrounding the piece. Our methods permit practically theoretical density of the article and minimal internal and external oxidation.

Further and importantly, hardened articles can be manufactured at a high production rate with excellent dimensional tolerances and ductility and hardness properties and without extra heat treatment due to the rapid cooling in the die combined with the deformation process especially for materials such as steels which can be hardened by martensitic transformation. Slow compaction or forging by the use of multiple blows of the plunger are unsuitable for the purposes of our discovery since the hot workpiece must be rapidly formed at the working temperature. This is one embodiment of our invention we have discovered that effective heating of electrically conductive di-electric lossy particles such as a green compact of metal powder or doped plastics such as plastics containing carbon particles, etc. may be accomplished by employing pulsating high frequency induction heating. We have found that the employment of continuous high frequency energy to a mass of metal particles for a period of time sufficient to reach proper hot working temperatures (e.g. 500 to 2500 F.) permits some of the electrically and thermally conductive metal particles to rapidly increase in temperature and become overheated and to transfer all or a substantial portion of this heat to the heat sensitive materials blended with the metal powders or to create grain growth and excessive oxidation. Excessive oxidation prevents the attainment of high density such as approaching theoretical density and damage the microstructure in the article to be formed. The use of continuous high frequency energy to heat the green compact will not permit the advantages of our discovery to be obtained.

The use of a high frequency energy for short periods of time such as from 0.1 second to a few seconds or more depending upon the mass and the distribution of mass in the compact permits elevated temperatures to be obtained in the metal powders without significant heat transfer to the nonelectrically conductive particles of the blend such as the dry lubricants or without excessive oxidation and uneven temperatures in the work piece. Self-lubricating particles such as calcium fluoride, molybdenum disulfide, tungsten disulfide, graphite, fine particle diamond powder, boron nitride, and other metal compounds are, with few exceptions, not electrically conductive under ordinary circumstances. The powdered metals of the matrix are electrically conductive and should have sufiicient resistivity so that it is a dielectric lossy material. The heat transfer of the heat created by the high frequency energy in the metal particles to the dry lubricants and to the ambient atmosphere is significantly reduced, when the high frequency is employed in a short-timed pulsating manner. This reduction in heat transfer permits dry lubricants and other materials normally degraded such as oxidized at conventional pressing temperatures to be incorporated into sintered articles without excessive decomposition. Further, our technique reduces reactions between the atmosphere and the metal matrix and permits impact forming of articles of high density. The use of this pulsing technique also gives rise to transient mechanical core stresses generated by the cyclic variation of the heat flow during plusing. These core stresses promote densification and increase the mechanical strength of the finished article.

The time of the pulses depends upon the nature of the materials employed, but we have found that relatively short periods of from about 0.5 to 3 seconds are often quite successful. The mass of powdered metal may be placed in a nonconductive mold and the mold surrounded by a water-cooled conductive helix of copper coil. High frequency energy of from 1000 cycles to 10 megacycles may be used, but normally 10 to 1000 kilocycles are employed. The power pulses may be of uniform time or vary in time depending upon the heat transfer characteristics of the mass and its distribution and the presence or absence of heat sensitive material. For example, softer metals like lead may require temperatures of only 200 C.. while nickel requires about 650 C. and tungsten about 2000 C. The metal powder mixtures with or without additional binder materials or additives may be compacted in a confining die under very high forging pressures of about 30,000 to 50,000 pounds per square inch or higher to form the hardened metal powder article. The article may be further aged and slowly or rapidly quenched in a fluid such as air, oil, water, etc. to further enhance the hardening or other characteristics of the article.

The mixtures, containing dry lubricants can be formed into a variety of improved antifriction articles such as porous powdered metal articles, having load bearing moving or nonmoving frictional surfaces where one or more of the surfaces or articles contain reservoirs or pockets of the dry lubricants or other material. The articles formed can be of any predetermined shape, size or characteristics and include brushes for electrical generators, contacting switches, gears, dashpots, valve stems, tubular elongated bearing sleeves, journal and thrust bearings, solid and tubular shafts, races, retainers, bushings, etc., or other articles such as those subject to frictional forces and having wearable destructible friction surfaces. The dry lubricant may be evenly dispersed through the metal matrix of the material. Where desired the lubricant may be incorporated only into particular areas of the article such as the surface areas of other areas of the article subject to particular severe frictional forces.

In one embodiment of the invention the matrix material comprises any electrical conductive material containing minor amounts of nonelectrically conductive heat sensitive materials. For example, powdered metals, metal alloys and metal-containing compounds capable of being compacted by conventional powder metallurgical methods may be used. The metals employed include those relatively soft metals, the hardest steel tool metals and metal alloys. The powdered metals employed can thus include those metals such as tin, lead, zinc, copper, nickel, iron, cobalt, aluminum, etc. and metal alloys containing trace or minor quantities of property modifying metal constituents such as copper, tin, zinc, lead, iron, chromium, cadmium, magnesium, palladium, platinum, rhodium, vanadium, nickel, cobalt, tungsten, molybdenum aluminum manganese, etc., and combinations thereof either with or without other additives and dry lubricants such as metal sulfides and selenides, boron nitride, graphite and the like.

The dry lubricants can be thoroughly admixed with the metal or metal alloy powder by hand or more efficiently by high-speed powder blending machines. Good results are commonly obtained by having the metal powder particles of larger average diameter than the dry lubricant powder with which it is admixed. The quantity of the dry lubricant to be admixed with the metal powder depends on the amount and degree of lubrication desired and the particular conditions under which the article is to be employed. Commonly the amount of heat sensitive non-electrically conductive materials employed will vary from about 1 weight percent to about 35 weight percent such as between 3 to 25 weight percent of the matrix. Generally in self-lubricating metal alloy compositions the tensile strength elongation, and compressive strength increase with decreasing amounts of dry lubricant, while as the amount increases the coefficient of thermal expansion and electrical resistivity increases and the hardness decreases.

The drawing shows a high frequency heating apparatus of our invention which includes: a high frequency RF generator such as a 20 kilowatt generator connected to power input source (not shown), a timer 12 and an input power regulator 14. The timer 12 permits RF energy to be generated in a timed cyclic manner as heretofore described. The input power regulator 14 or other variable means is used to control the power output of the generator 10. In practice the input power control 14 can be primary of the generator transformer to control the voltage to primary and thus act as a variable transformer.

Our apparatus includes a metal die 16 resting in a lower support base 18 having extending support 20 to an upper support base 22 containing a reciprocating slidable plunger 24 vertically extending and in registry with the die 16. The forward face of the plunger contains a feeler 25 whose shape and size is characterized by the article to be formed with the upper plunger face 28 adapted to be struck by a pneumatic drop hammer or other means of applying a large downward force to the plunger 24-. Disposed between the forward or forming face of the plunger 24 and die 16 and in registry with the feeler 26 and the internal face of the die 'is a green powdered metal workpiece 30 shown in a ring shape. The workpiece 30 is supported at its lower periphery by molybdenum alloy spring loaded fingers 32 and 34 which swing downwardly and out of the way of the workpiece when the workpiece is forced downwardly into the die 16 by the plunger 24. The fingers 32 and 34 are pivoted at 36 and 38 and are tensionally held by springs 40 and 42 in a horizontal workpiece supporting position with the opposite ends of each finger resting on stops 44 and 46. Surrounding the workpiece 30 is a hollow helix 48 of copper tubing or other RF conductive material with the longitudinal axis of the helix similar to the workpiece 30. The coil 48 is electrically connected through leads 50 to the output of the RF generator. In operation water or some other cooling medium is often circulated through the helix 48 to prevent overheating.

In operation the workpiece 30 is inductively heated by the application of high frequency energy to helix 4 8 for short periods of time by the periodic actuation of the RF generator 10 by timer 12. When the workpiece 30 has reached the desired hot even working temperature, the upper face 30' of plunger 24 is stuck and the plunger rapidly forced downwardly with great force. The feeler 26 enters the annulus of the workpiece and the plunger 24 carries the heated workpiece into the die 16 where it is compacted. The downward motion of the plunger 24 and the workpiece 30 forces fingers 32 and 34 downwardly and out of the way. In the die 16 the workpiece 30 is compacted and rapidly cooled by the metal die in a single operation. In this manner steel parts having a Rockwell C hardness of 55 or better can be obtained.

For example, 52100 steel powder, annealed to a soft condition, was pressed at 35,000 .p.s.i. into rings of 0.800" O.D., 0.5" 1.1)., and 0.25" thickness. These rings were placed inside a working RF coil of 1.25" diameter having two turns, placed between the plunger and die in a die set. The plunger had a feeler with the same dimensions as the inner diameter of the rings. The rings were heated very quickly, using 70% power setting (rated output 20 kw.) on the RF generator. When the temperature reached 2000 F. (average time 3.2 sec. using three /2 sec. pulses), a 150 pound metal slab was dropped on top of the plunger, forcing the plunger into the ring and pressing the ring down into the die. The free drop of the metal slab was 10 inches. The resulting rings, after ejection from the die set, had a Rockwell C hardness of 55, 99%+ theoretical density and very uniform pore-free microstru-cture. The rings had an average thickness of 0.18" after forging.

As described, the use of pulsed R F energy in combination with rapid single-stroke forging techniques, permits metal powder articles of exceptional density and hardness to be prepared. Metal powdered articles formed by our method also have a uniform pore-free microstructure. Although our invention has been described, in particular as to RF energy means, any means of pulsed heating and rapid coining or forging of the workpiece may be employed.

What we claim is:

1. In the method of forming a powdered metal article which includes the steps of forming a .green compact workpiece comprising electrically conductive particles and heating the workpiece by subjecting the workpiece to electrical energy the improvement which comprises: pulsating the electrical energy for very short time periods and for a sufiicient time to permit the workpiece to be heated to an even working temperature without excessive reaction with the atmosphere; and forming the hot workpiece in a single high impact stroke operation into an article of high density.

2. A method of forming a powdered metal article as described in claim 1 in which the very short time periods are from 0.1 to 3 seconds.

3. A method of forming a powdered metal article as described in claim 1 in which the workpiece contains a self-lubricating amount of non-conductive heat sensitive dry lubricant and the pulse time is selected to inhibit degradation of the lubricant.

4. A method of forming a powdered metal article as described in claim 1 in which the hot workpiece is quickly formed and quenched in heat exchange relationship with the die.

5. In the method of forming a powdered metal article which includes the steps of forming a green compact workpiece comprising powdered metal and heating the workpiece by subjecting the workpiece to high frequency electrical energy the improvement which comprises: pulsating the high frequency electrical energy for very short time periods of from about 0.1 to 3 seconds and for a suflicient period of time to bring the workpiece to an even working temperature in a heating position; rapidly transferring the hot workpiece to a forming position; and forging the hot workpiece by a single stroke high-impact operation into a high density article.

6. A method of hot impact consolidation of metal powders which method comprises:

supporting a green compact workpiece comprising powdered metal in a heating position between and in registry with a die having a die cavity and a die plunger;

placing the workpiece in an inductive heating relationship with a high frequency coil;

heating the workpiece to an even impact-hot working temperature with pulses of high frequency energy in the coil of from 0.1 to 3 seconds;

rapidly transporting the hot workpiece into the die cavity; and

forging and cooling the hot workpiece into a high density article by a single high impact stroke of the die plunger into the die cavity.

7. A method as described in claim 6 which includes withdrawing the support of the workpiece as the die plunger contacts the workpiece.

References Cited UNITED STATES PATENTS 3,248,215 4/1966 Bonis et a1. -226 CARL D. QUARFORTH, Primary Examiner. BENJAMIN R. PADGETT, Examiner.

A. I. STEINER, Assistant Examiner. 

5. IN THE METHOD OF FORMING A POWDERED METAL ARTICLE WHICH INCLUDES THE STEPS OF FROMING A GREEN COMPACT WORKPIECE COMPRISING POWDERED METAL AND HEATING THE WORKPIECE BY SUBJECTING THE WORKPIECE TO HIGH FREQUENCY ELECTRICAL ENERGY THE IMPROVEMENT WHICH COMPRISES: PULSATING THE HIGH FREQUENCY ELECTRICAL ENERGY FOR VERY SHORT TIME PERIODS OF FROM ABOUT 0.1 TO 3 SECONDS AND FOR A SUFFICIENT PERIOD OF TIME TO BRING THE WORKPIECE TO AN 